Or you could use the already included draft message setup.

Tools-Account Settings-Copies and Folders to define the folder

and

Tools-Options-Composition-General to set the auto save options

No menu bar with Tools? Press alt or F10 to make it appear.

The autosave to draft was already set to 5 minutes. I've reduced it to every minute. I suspect because the attach (of whatever it was trying to attach) didn't complete, only an early copy of the reply was in drafts. Since drafts didn't contain a recent version of the reply, my question still stands: is there some other location where TB is saving incomplete work?

Apart from Sent and Drafts, another useful folder to know about is Outbox, (under Local Folders) which is where Thunderbird places messages after you hit Send and where they wait until it is able to connect and send them.

But, by definition, only complete messages should appear here, so this is not an answer to your last question,

I don't know of anywhere other than the specified Drafts folder that Thunderbird would use. I suspect your problem is a recurring one that involves saving messages to Sent, and this process failing when there are attachments or inserted images. It's particularly aggravating when you haven't specifically attached anything yourself, and it's trying to re-attach some previous inclusion. In my experience, other people's graphics in their signatures are a frequent problem.

<rant>I'm quite embarrassed that my employer has just issued a formal signature block for us all to use, which was clumsily put together using Word, with heaps of MSO metadata and three largish graphics. This ugly signature is often bigger than the email message text, and certainly is the usually the biggest part of a message when you consider the number of bytes it occupies. </rant>

Zenos - Bingo! Thanks for that HELPFUL REPLY. There are graphic signatures in the previous incoming messages in the thread that triggered the query here. I noticed while redoing my most recent reply that the auto-save every 1 minute started showing "attaching" in the bottom of the window. Rather than continue editing, or attempting to send, I selected File>Send Later. This produced a new file in the Outbox folder (under Local Folders, as you mentioned).

In my experience, using Send Later when there is a problem with Send gives the system the opportunity to fix whatever is amiss.

So the solution here, when "attaching..." shows up during an auto-save operation, is also to use Send Later. And if there is more to be added to one's reply, right click that file in the Outbox and select "Edit as New."

Be careful: if you use Send Later multiple times with the same message followed by Edit as New, you will have multiple entries in Outbox with the same name, the most recent being in bold. To keep things sorted out, I delete the other versions. (It would be neat if TB would timestamp them for easier identification). Also note that you have only the version you want sent when you later send it via File> send unsent messages.

Re your rant - would your employer let you take a screen shot of that messy signature and attach that instead of all the messy MSO junk?

Actually I reverse engineered the signature block into a Thunderbrd HTML signature, using a table to manage the layout, and PNGs for the graphics instead of the fuzzy amateurish jpegs that were provided. And all the MSO cruft and custom Microsoft-only fonts have gone. ;-)

The Word-generated signature block fails spectacularly in a narrow display, or in environments such as the mail client on an Android device. The use of a table forces a minimum width, but stops the content from wrapping. You now have to scroll sideways to see it all in my version, but at least the relationship between the various parts is maintained.

To my mind, dong the whole thing as a graphic would be worse; it would be bigger and non-parseable, so to use the included information, such as the company website URL or my phone number or the included linkedin and twitter links, users would have to manually read and copy the relevant bits of text.

Even though the internals are cleaner in my version, the whole thing is still IMHO ugly and antisocial; it is, by dint of its design, intrusive and too big.

Anyhow, glad to hear I've managed to help you to ease some of your problems.